Currently, holographic gratings and image storage devices employ inorganic photorefractive crystals. While such crystals perform adequately to provide image storage functions, they are expensive and hard to grow. Liquid crystal films posses similar performance capabilities, and exhibit a number of advantages. In particular, liquid crystal films are highly optically non-linear and, to obtain efficient grating diffraction efficiencies, require films of only a few to tens of microns thickness. Bulk conventional photorefractive crystals require dimensions of several millimeters to accomplish such functions. Further, liquid crystal films are particularly suited to large area fabrication whereas conventional photorefractive crystals are much smaller. Liquid crystal-based devices also exhibit certain unique characteristics: e.g., compatibility with microelectronic and integrated optic circuit technologies; very broad bandwidths; response speeds in millisecond to submillisecond regimes; and very low cost (on the order of two orders of magnitude less than inorganic photorefractive crystals).
Different groups are now developing polymer-based photorefractive liquid crystal materials. Gibbons et al. report on a method for inducing a persistent memory state in polymer-based liquid crystal films which include a dopant dye. The liquid crystal film is exposed by employing a linearly polarized light having a wavelength in the absorption band of the dopant dye within the liquid crystal medium. The linearly polarized light is transmitted through a mask having a pattern, or the beam is scanned and modulated to create the pattern. The beam pattern results in localized reorientations of the liquid crystal domains, which reorientations remain when the incident optical energy is removed. See Gibbons et al. "Surface-Mediated Alignment of Nematic Liquid Crystals with Polarized Laser Light", Nature Vol 351, Pages 49, 50, May 2, 1991 and U.S. Pat. Nos. 4,974,941 and 5,032,009. U.S. Pat. No. 5,073,294 to Shannon et al. details further work on the polymer-based photorefractive material described by Gibbons et al. and employs the same process for exposure of the liquid crystal media.
Chen et al. in "Real-Time Holography in Azo-Dye-Doped Liquid Crystals", Optics Letters, Vol 17, No 6, pages 441-443, Mar. 15, 1992, describe the use of azo-dye-doped nematic liquid crystal films for providing a holographic storage media. Chen et al. indicate that the photosensitivity of the azo-dye molecules and the large birefringence of the liquid crystal molecules provide an improved holographic storage medium. Chen et al. introduce a sodium salt, 4-dimethylaminoazobenzene-2'-carboxylic acid (an azo-dye molecule) into 4'-pentyl-4-biphenylcarbonitrile (PCB), a nematic liquid crystal host. A pair of argon laser beams record a hologram in the sample cell.
In each of the above-reported prior art teachings, a highly absorbing dye-doped liquid crystal film is employed to store a spatial optical effect. Each of the reported films is highly absorptive causing incident light to be substantially absorbed and to create large temperature fluctuations therein. They also possess low diffraction efficiencies on the order of 0.001. Furthermore, the dye-doped materials require writing times on the order of several tens of minutes.
Accordingly, it is an object of this invention to provide an improved method for inducing a persistently retrievable image in a liquid crystal film.
It is a further object of this invention to provide an improved method for rapidly inducing persistently retrievably optical storage into a doped liquid crystal media.